A polymer coating imparts two important functions to the underlying substrate: the aesthetic function gives the substrate a good appearance while the protective function protects the substrate from mechanical and chemical damage. Due to routine wear and tear, surface scratches are generated and micro-cracks develop and eventually lead to macroscopic damage, which results in the coating losing its aesthetic and protective functions. Corrosion protection coatings are especially intolerant to crack formation because it will expose the underlying metal to a corrosive environment, thereby shortening the service life of the coating. The use of a self-healing coating will increase the operational life of coatings and eliminate the need to frequently repaint or replace damaged coatings. In addition to a huge cost saving, this results in significant savings in energy used in fabricating the coating materials and applying the coatings.
When a polymer coating is mechanically damaged, it is either plastically deformed at the surface (such as a blunt indent or a light scratch), or cracked (such as a cut made with a sharp blade). The majority of existing self-healing technologies do not target both forms of damage. One is neglected at the expense of the other. An example is the polyurethane based self-healing coating developed by Bayer Material Science for the automotive industry [Benthem, R.; Ming, W.; With, G., Self Healing Polymer Coatings. In Self Healing Materials, 2007; pp 139-159;]. The coating utilized the shape memory effect (termed as “reflow effect”) to recover plastic deformation, for example the scratch from a car wash. There is no “self-healing” in the case of cracking or micro-cracking due to the lack of chemical or physical forces to “re-bond” the newly generated crack surfaces.
Cho et al. [Cho, S. H.; White, S. R.; Braun, P. V., Self-Healing Polymer Coatings. Advanced Materials 2009, 21, (6), 645-649.] developed a coating containing encapsulated healing agents and catalysts that specifically tackles crack healing/re-bonding, based on their previous success with bulk self-healing polymers using the same strategy [White, S. R.; Sottos, N. R.; Geubelle, P. H.; Moore, J. S.; Kessler, M. R.; Sriram, S. R.; Brown, E. N.; Viswanathan, S., Autonomic healing of polymer composites. Nature 2001, 409, (6822), 794-797.]. A surface crack made by hand scribing with a sharp razor blade ruptures the capsules and triggers the crosslinking of the released healing agent to seal the crack. This strategy is only applicable to healing cracks, since the formation of a crack is necessary to rupture the capsules where no capsules are likely to be ruptured (therefore no healing agent released) during plastic deformation. A similar methodology uses hollow fibers [Dry, C., Comp. Struc., 1996, 35, 263-269.] or interconnected microchannels [Toohey, K. S.; White, S. R.; Sottos, N. R., Self-healing polymer coatings. Proc. 2005 SEM annual conference] to store the healing materials. Fabrication of a microvascular network is a challenge, which limits its application.
Besides polymerization based healing, reversible chemistry based on Diels-Alder reaction, hydrogen bonded supramolecular network, and ionic liquids have been utilized for producing healable polymer materials. All these are limited to a narrow set of less commonly used polymers, rendering widespread utilization of the technology unlikely. A unique approach among these methods is a thermoset/thermoplastic mixture that offers crack sealing capacity when the material is heated, and the thermoplastic is able to diffuse across the crack boundary [Jones, F and Hayes, S. A., “Self Healing Composite Material”, WO 2005/066244 A2.]. However, the only driving force for crack sealing is chain diffusion, and crack closing is not part of the method. Most recently, researchers from University of Southern Mississippi invented a polyurethane based self-healing material by mixing chitosan into the compound. Upon exposure to UV light, chitosan rods are broken up and bond to each other across the damaged area. A drawback of this technology is that repeated healing is not possible.